1. Field of the Invention
The present invention relates to a field emission type electron gun useful for e.g. electron beam exposure devices or electron beam devices for high brightness.
2. Description of the Prior Art
A typical conventional field emission type electron gun is illustrated in FIG. 1, wherein a Butler's anode is employed as the anode. A filament 4 for heating a first anode 2 to generate a gas is disposed obliquely above the first anode 2. A cathode 1 and a second anode 3 are housed in a vacuum container 5 together with the first anode 2 and the filament 4.
In such an electron gun, when the first anode 2 is heated by the filament 4, the temperature of the surface of the first anode 2 is not so high, whereby generation of gas from the first anode is insufficient. As a result, although adsorbed molecules (e.g. water) having a small binding energy can be removed, oxygen atoms having strong bond strength will be retained on the surface of the first anode 2. Accordingly, with an increase of the emission current, there will be an increase in the probability of the liberation or ionization of the adsorbed gas caused by excitation of electrons. The liberated molecules, atoms or ions are likely to be absorbed or impinge on the surface of the cathode 1, and thus tend to change the work function of the electron emitting surface and to change the shape of a tip. Thus, there has been a difficulty that the emission current tends to be unstable.
In order to eliminate the above-mentioned difficulty, there has been proposed an electron gun as disclosed in U.S. Pat. No. 4,596,942. As shown in FIG. 2, an anode is formed by a horn-shaped plate in which a central opening portion is raised instead of the conventional Butler's anode, and a filament 4 is placed immediately before the first anode 2 of the horn-shaped plate in a symmetrical manner so as to serve as a gas expelling member and an anode. However, this electron gun has the problem that the process of expelling of gas cannot be completed, although some gas expelling can be attained through the raised portion at the center of the anode.
The inventors of the present application have got an idea, after further study, that an anode is formed to have an elongated shape in which a through hole is formed at its center, whereby gas is effectively and thoroughly expelled from the anode. However, when an anode having an elongated shape is used, some electrons emitted from the cathode may take orbits out of the anode to impinge on the base portion of the anode (as shown in FIG. 3(a)) to thereby cause the liberation or ionization of adsorbed gas due to excitation of the electrons. As a result, there produces an unstable emission current.
The present invention has been attained by finding that a stable emission current can be obtained by providing a control electrode near an anode or between an anode and a cathode and by rendering the electric potential of the control electrode to be negative to that of the anode, whereby electrons emitted from the cathode are converged at the top end of the anode by the control electrode and impinging of the electrons on the lower base portion of the anode is prevented. Further, the filament sufficiently functions to expel gas from the top portion of the anode.